The prior art magnetic actuators are used in a variety of magnetic isolation, pointing and bearing suspension systems. These prior art actuators are used to apply an electromagnetic force to a body without physically contacting it. As current is applied through a stator coil, a force-producing flux is generated, which flows through a magnetic circuit. Ideally, a magnetic actuator would apply a known and repeatable force, for a commanded current level through the coil. In reality, however, there is an error or difference between the commanded and applied forces, which translates into system level errors. Minimizing this error is of paramount importance to precision pointing and isolation systems.
In the prior art actuators, different active control systems or devices have been employed to linearize the response of magnetic actuators. A few equations will be useful in describing some methods of controlling the magnetic armature force: ##STR1##
F=electromagnetic force
A=cross-sectional area enclosed by the magnetic circuit
u.sub.o =permeability in a vacuum (approximately the same through air)
B=magnetic flux density ##STR2##
N=number of coil turns
I=current through electromagnet coil
g=gap across magnetic interface
*=times
Substituting equation 2 into equation 1 yields: ##STR3##
From the equation above, force is proportional to the square of flux density, which in turn is proportional to the quotient of current and gap.
A first prior art magnetic actuator has a force control device, which is the most direct and accurate method of controlling the force applied to the armature, and which uses an accurate force measurement unit, such as a quartz force crystal, as the feedback element. Although very linear and accurate, this device requires the use of a relatively fragile sensor bridging the gap between the stator and armature interface. This requires that the sensor be protected during high acceleration, and also creates a spring mass mode, which limits the actuator bandwidth. This device is also the most expensive, and requires complex support electronics.
A second prior art magnetic actuator has a current-gap control device, which uses the relationship of equation 3 to control the magnetic actuator force. Proximeters measure the gas between stator and armature sections, and a current sensor measures the current through the magnetic actuator coil. Advantages of this device are the relative ease of implementation, and readily available sensors. The major disadvantage of this device stems from the fact that there are deviations from the ideal current-gap to flux relationship. A precision magnetic actuator, using current-gap control, requires a magnetic material with nearly a linear flux-current relationship. To attain this linearity, the magnetic material must exhibit low hysteresis, which requires careful and costly material processing. Also, a low-hysteresis material saturates at a relatively low flux density, necessitating a larger volume and weight actuator for a given force, relative to a high-hysteresis material.
A third prior art magnetic actuator has a flux control device with a flux sensor. Since force is proportional to flux squared (equation 1), a control loop using a flux sensor closes around hysteresis, actively controlling this nonlinearity. This device, uses high-hysteresis material, thereby reducing the cost and weight of the actuator. The disadvantage of this device is that it requires a more complicated sensor than the current-gap control device. Biasing techniques are used to operate a differential magnetic actuator within a linear operating region. The bias technique solves the problem associated with controlling a parameter proportional to flux squared with a sensor whose output is proportional to flux. The disadvantage of the bias technique, is that power must be dissipated through the actuator coils even when no differential force is required. To reduce the quiescent power of the device, a different controlling concept is used, taking the square root of the force command and delivering it as a flux control loop command. This requires a nonlinear element (analog multiplier) in an analog control system, or a square-rooting algorithm in a digital controller. An analog square-rooter reduces the overall accuracy of the actuator system, by the nonlinearity of the analog device. This problem is less of an issue with a digital controller, but does require additional computational lag to perform the square-rooting function.